It's a regional thing; they seem to be common here in Michigan. It seems to be proportional to the degree of temperature swings? They're not great if the ground

It's a regional thing; they seem to be common here in Michigan. It seems to be proportional to the degree of temperature swings? They're not great if the ground and air are always warm or always cold. We have winter lows below 10F/-10C and summer highs above 85F/30C. But the ground temperature is a relatively uniform 50F - useful for both heating (less important, because natural gas is cheap and efficient) and cooling. It's not so useful when the average temp is above your desired room temp.